This invention relates to a novel polymer. It further relates to a method for preparing a polymer. In another aspect, the invention relates to a novel method for altering polymer structure using radiation.
Polyethylene is a commercially-important synthetic thermoplastic material. Its commercial importance is enhanced by increasing the variety of forms of polyethylene having different physical properties and therefore different end-use applications. The scientific study of polyethylene has provided information on the relationship between the molecular structure of polyethylene and its physical properties. It is known that molecular weight, density and molecular weight distribution are properties of polyethylene homopolymers and copolymers which significantly affect the properties of the polymers and determine the end uses for which they are suitable. It is also known that the molecular structure of polyethylene affects the properties of the polymer, and work has therefore been directed to controlling and altering the molecular structure of polyethylene to produce new forms having improved physical properties.
Polyethylenes produced commercially via high pressure, free radical processes have densities around 0.92 g/cc and are referred to as "low density" polyethylenes. It is known that the polymer molecules produced by such processes have appreciable quantities of ethyl, butyl and amyl branches and relatively few long chain branches in comparison. Polyethylenes having densities of about 0.96 g/cc are called "high-density" polyethylenes and can be prepared using either titanium or chromium based catalysts. The molecules are usually linear, although the properties of some high density polyethylenes suggest the presence of long chain branching at a level one to two orders of magnitude below that found for low density polyethylenes prepared by a high pressure process. The use of high sensitivity NMR spectrometers in combination with careful analytical methods enables the characterization of polymer molecular structure, molecular weight (M.sub.w and M.sub.n) and molecular weight distribution (M.sub.w /M.sub.n).
The ability to characterize molecular structure and to determine the relationship between molecular structure and polymer properties has led polymer scientists to attempt to devise methods for preparing polymers meeting specific structural or end-use needs.
It would be desirable to produce a polymer characterized by long chain branches but without the accompanying short chain branches produced in the high pressure ethylene polymerization process. It would be desirable to be able to control the degree of long chain branching of a polymer and thus produce polyethylene having a predetermined set of physical characteristics.
It is therefore an object of the present invention to produce a novel polymer. It is a further object to provide a method of controlling or altering the molecular structure of a polymer. It is an object of one embodiment of the invention to produce a polymer the molecules of which have a relatively large number of long chain branches compared to short chain branches.